1. Field of the Invention
The present invention relates to an optical pickup, and more particularly, to an optical pickup which is applicable to an optical disc device in which an optical disc is accessed by an optical system having a high numerical aperture. The present invention aims to effectively avoid deterioration of the characteristics due to the displacement of an objective lens and an aberration correcting mechanism, by holding the objective lens and the aberration correcting mechanism so that they move together.
2. Description of the Related Art
Hitherto, compact discs and mini discs used for music, DVDs (Digital Versatile Discs) used, for example, for movies, MOs and CDs-R/W used for data recording in computers have been used as optical discs.
In optical disc devices for accessing such optical discs, the wavelength of a laser beam to be applied to the optical disc is decreased and the numerical aperture of an optical system is increased in order to reduce the size of the beam spot formed on the information recording surface of the optical disc, and to further increase the recording density.
However, when an optical disc is accessed with an objective lens having a high numerical aperture, spherical aberration is caused by variations in the thickness of a light transmitting layer of the optical disc. Furthermore, it is usual to increase the numerical aperture of the objective lens by using a so-called two-element objective lens. In this case, spherical aberration is also caused by variations of the distance between the lenses. When the recording capacity is increased by forming the information recording surface of the optical disc of multiple layers, spherical aberration is also caused by these multiple layers.
For this reason, for example, Japanese Unexamined Patent Application Publication No. 2000-131603 proposes an aberration correcting mechanism for correcting such aberrations, in which a lens is interposed between an objective lens and a laser light source so as to correct the wavefront of a laser beam.
In such aberration correction, however, the objective lens and the aberration correcting mechanism may be displaced from each other. In this case, the characteristics are inevitably deteriorated by comatic aberration in the optical pickup.